Anode assembly for selective corrosion protection of metal parts in concrete

ABSTRACT

Disclosed is an anode assembly for the corrosion protection of metal parts embedded in concrete. An ion-conducting material is placed between the metal part that is to be protected and the anode, which ion-conducting material exhibits higher ionic conductivity than the surrounding concrete, thus directing the protective current specifically towards the metal part. A galvanic sacrificial anode may be provided, made, e.g., from zinc and its alloys or aluminum and its alloys. The purpose of the material with higher ionic conductivity than the surrounding concrete is to selectively direct the protective galvanic current towards the metal part that is to be protected. The selective enhanced corrosion protection is especially beneficial for the protection of metal parts that are highly important for the structural integrity of concrete members, such as assembly of pre- or post-tensioning of concrete members, such as anchor-heads. The ion conducting material exhibits 20%, and more preferably 50%, higher conductivity than the surrounding concrete. A suitable ion-conducting material that exhibits ion-exchange properties may comprise tecto-alumo-silicate materials. The anode may be placed in close contact to or may be embedded into the ion-conducting material as well.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/689,435 titled “Anode Assembly for Selective Corrosion Protection of Metal Parts in Concrete,” filed Jun. 25, 2018 by the inventors herein, which application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to the corrosion protection of metal parts embedded in concrete, and more particularly to systems and methods employing ion-conducting material to direct corrosion-protective current from an anode to a concrete embedded metal part.

BACKGROUND

Corrosion of steel reinforcement is one of the most significant causes of elevated maintenance and repair costs and, subsequently, of the shortening of the useful life of steel-reinforced concrete structures. Corrosion of the steel reinforcement is caused by the penetration of chlorides into the covering and/or by carbonation of the concrete covering. Components of civil engineering structures, such as bridges, tunnels, parking garages, etc., that are frequently exposed to de-icing salt, and structures such as harbor installations, bridges, apartment buildings, etc., that are close to the sea and are exposed to sea salt, are endangered and affected by chloride-induced corrosion of the steel reinforcement, which corrosion is caused by chlorides penetrating into the concrete. In such components, in the event of damage, the chloride-contaminated concrete has to be removed down to beyond the steel reinforcement and replaced by new fresh concrete or repair mortar. However, this method of repair is very complex, labor-intensive and costly.

An alternative to conventional repair of built structures that have become endangered by the corrosion of the steel reinforcement is cathodic corrosion protection (CCP), as described, for example, in EP1068164B1, which method has been in use for approximately 30 years. An alternative and/or supplementary measure to CCP which is also used is galvanic corrosion protection (GCP), as described, for example, in AT 1344/2004, EP1135538, EP0668373 and in U.S. Pat. No. 4,506,485. The effect of GCP is based on the formation of a galvanic element between a sacrificial anode and the steel reinforcement of a concrete structure, with the concrete acting as the electrolyte. The anode materials used in such GCP systems are typically zinc and alloys thereof, and less commonly aluminum and alloys thereof. The anode is typically installed either on the concrete surface or in holes drilled in the component that is to be protected. Such a galvanic anode system is described, for example, in U.S. Pat. Nos. 6,022,469, 6,303,017, 6,193,857. A galvanic anode that is installed on the concrete surface by embedding the anode in an embedding zinc activating binder or embedded into the concrete as an anode assembly is described in U.S. Pat. No. 8,394,193B2 and in GB patent out of EP 2 313 352. Galvanic surface anodes are described in U.S. Pat. No. 7,851,022 and in EP 05768008.4.

The specifications of each of the foregoing references are hereby incorporated by reference in their entireties.

Structures that are exposed to high loads or whose design demands slim dimensions are usually reinforced with pre-stressed or post-stressed tendons. Such assemblies typically consist of two anchor heads between which the tendon strands are tensioned. The great advantage of tensioned concrete members are their ability to sustain high loads, as well as the slim structures that may be realized through their use. Usually, only a few tensioning assemblies are installed in a concrete member. The structural integrity of the post- or pre-tensioned concrete member relies essentially on the tensioning assembly. Therefore, corrosion of the parts of the tensioning assembly (such as the anchor head, wedges, tendons, and strands) may lead to catastrophic failure of the structure. For that reason, corrosion protection of the tensioning assembly, especially in post-tensioned structures, is essential. Usually, if only a few tensioning assemblies—e.g., in bridges, five tendons over the entire bridge deck—are damaged by corrosion, the whole structure loading capacity could be compromised and in need of extensive repairs or replacement. For concrete members, e.g., balconies, the failure of only one part of the tensioning assembly necessitates the replacement of the concrete member.

Therefore, the selective corrosion protection of the parts of the tensioning assembly in such concrete structures is essential for assuring the projected service time of the structure and the concrete members.

SUMMARY OF THE INVENTION

Disclosed herein is an anode assembly for the corrosion protection of metal parts embedded in concrete. An ion-conducting material is placed between the metal part that is to be protected and the anode, which ion-conducting material exhibits higher ionic conductivity than the surrounding concrete, in turn directing the protective current specifically towards the metal part. In accordance with certain aspects of an embodiment of the invention, a galvanic sacrificial anode is provided that is made from, e.g., zinc and its alloys or aluminum and its alloys. The purpose of the material with higher ionic conductivity than the surrounding concrete is to direct and selectively focus the protective galvanic current towards the metal part that is to be protected. The selective enhanced corrosion protection is especially beneficial for the protection of metal parts that are highly important for the structural integrity of concrete members, such as assemblies that include pre- or post-tensioning of concrete members, such as anchor-heads. In accordance with certain aspects of an embodiment of the invention, the ion-conducting material exhibits 20%, and preferably 50%, higher conductivity than the surrounding concrete while at the same time exhibits no adverse effects on the steel that is to be protected. In accordance with still further aspects of an embodiment of the invention, an ion-conducting material is placed that exhibits ion exchange properties, such as tecto-alumo-silicate materials. The anode may be placed in close contact to or may be embedded into the ion-conducting material.

In accordance with certain aspects of an embodiment of the invention, a system for the corrosion protection of metal parts embedded in a concrete member is provided, comprising: a metal part embedded in the concrete member; a first anode; and an ion-conducting material between the metal part and the anode, the ion-conducting material exhibiting higher ionic conductivity than concrete embedding the metal part, and wherein the ion-conducting material is configured to direct a corrosion-protective current towards the metal part.

BRIEF DESCRIPTION OF THE FIGURES

The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized. The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements, and in which:

FIG. 1 is a sectional perspective view of a prior art concrete tensioning system in which embodiments of the invention may be applied.

FIG. 2 is a sectional schematic view of an anode assembly for selective corrosion protection of metal parts in concrete in accordance with certain aspects of an embodiment of the invention.

FIG. 3 is a section schematic view of an anode assembly for selective corrosion protection of metal parts in concrete in accordance with further aspects of an embodiment of the invention.

FIG. 4 is a section schematic view of an anode assembly for selective corrosion protection of metal parts in concrete in accordance with still further aspects of an embodiment of the invention.

FIG. 5 is a section schematic view of an anode assembly for selective corrosion protection of metal parts in concrete in accordance with still yet further aspects of an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention may be understood by referring to the following description and accompanying drawings. This description of an embodiment, set out below to enable one to practice an implementation of the invention, is not intended to limit the preferred embodiment, but to serve as a particular example thereof. Those skilled in the art should appreciate that they may readily use the conception and specific embodiments disclosed as a basis for modifying or designing other methods and systems for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent assemblies do not depart from the spirit and scope of the invention in its broadest form.

Descriptions of well-known functions and structures are omitted to enhance clarity and conciseness. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced item.

The use of the terms “first”, “second”, and the like does not imply any particular order, but they are included to identify individual elements. Moreover, the use of the terms first, second, etc. does not denote any order of importance, but rather the terms first, second, etc. are used to distinguish one element from another. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

Although some features may be described with respect to individual exemplary embodiments, aspects need not be limited thereto such that features from one or more exemplary embodiments may be combinable with other features from one or more exemplary embodiments.

For experts knowledgeable of the state of the art of electrochemical corrosion protection—e.g., cathodic corrosion protection and/or galvanic corrosion protection—it is commonly known that the degree of protection depends on the amount of protective current arriving at the metal part that is to be protected. One of the parameters determining the amount of current arriving at the metal part that is to be protected is the resistivity, and more particularly the electrolytic conductivity, of the material placed between the anode and the metal part that is to be protected, which material serves as the electrolyte. Concrete is commonly used as the electrolyte for the cathodic protection of steel in concrete in civil structures. However, the electrolytic resistivity of concrete depends strongly on the humidity of the concrete and may range from about 100 Ohm.m in humid concrete (95% rh) up to several KOhm.m in dry concrete (50% rh) [1], covering a range of 3 orders of magnitude. Concrete exposed to variable weathering conditions may therefore show locally and over time large variations in concrete resistivity. Therefore, metal parts embedded into the concrete, such as an assembly for pre- or post-tensioning of concrete members consisting of an anchor head, wedges and steel tendons tensioned between the anchor heads, are difficult to reliably and durably protect from corrosion using electrochemical techniques. An exemplary prior art tensioning system is shown in FIG. 1, depicting a concrete structure (shown generally at 10), an anchor plate 12 embedded in concrete structure 10 and receiving an anchor head 14, which anchor head 14 receives tendons 16 which, in turn, provide tension to concrete structure 10. A pocket or opening 18 is formed in the concrete structure 10, to which a grout cap is typically applied to finish the installation.

Usually, if one of the assemblies shown in FIG. 1 fails, e.g., due to corrosion, then the concrete member 10 may suffer irreparable damage rendering it unusable until extensive repairs or replacement are undertaken. The most aggressive type of steel-corrosion is initiated by ingress of chlorides that are difficult to be sealed off. The most economic and well-established corrosion protection techniques are based on electrochemical corrosion protection techniques such as cathodic protection (CP). However, the size of, e.g., anchor heads made from steel may be as small as 2-3 inches in diameter. Therefore, small local variations of concrete resistivity may lead to local lack of corrosion protection. While this might be less crucial for the protection of mild steel reinforcement that is available in large quantities within reinforced concrete structures, corrosion on specific elements of pre- or post-stressed reinforcement elements could prove to be catastrophic to the structure, especially at the anchor area.

One may apply high currents or high voltages to assure that there is sufficient current flowing towards, e.g., the anchor heads of post-tensioned concrete structures. However, these parts are sensitive to hydrogen embrittlement. Hydrogen embrittlement may only be avoided with high expenditure, involving use of a high number of sensors (reference cells) and expensive control and regulation equipment and software.

Another option is corrosion protection with sacrificial galvanic anodes. Using zinc anodes, conditions of hydrogen evolution and subsequent hydrogen embrittlement may be avoided due to the maximum polarized potential of the steel. However, galvanic corrosion protection systems do not allow the adjustment of the applied current. The applied voltage is given by the system, and with concrete being the electrolyte between the galvanic anode and the metal parts that are to be protected, the current output is controlled by the concrete resistivity. Potentially, galvanic protection would be very suitable for protecting metal parts embedded into the concrete (such as, e.g., anchor heads of pre- and post-tensioning systems) if one can assure that enough current is reaching the sensitive parts of the tensioning system.

In accordance with certain aspects of the invention, a system and method are therefore provided to assure the reliable and durable protection of metal parts that are highly important for the structural integrity of concrete members, such as assemblies of metal parts for the pre- or post-tensioning of concrete members (e.g., anchor-heads). That system and method include placing an ion-conducting material that exhibits a significantly higher ion conductivity than the surrounding concrete between the anode, preferentially a galvanic anode, and the metal part that is to be protected. Preferably, the ion conductive material has an electrolytic conductivity that is at least 20%, and more preferably 50%, higher than that of the surrounding concrete, and whose dependence of the electrolytic conductance has a significantly lower dependence on ambient humidity. In accordance with certain aspects of an embodiment of the invention, suitable materials include, e.g., electrolyte gels based on poly-ethylene oxide (PEO), poly acrylonitrile (PAN), poly methyl methacrylate (PMMA), and poly vinylidene fluoride (PVdF), or materials with ion exchange properties such as ion exchange resins. An ion exchange resin on expanded poly-styrene basis and a PEO-based electrolyte gel proved to be suitable. However, organic materials tend to have a limited service time and weathering resistance. Suitable ion-conducting materials are also described in U.S. Pat. No. 7,851,022 and in EP 05768008.4. An inorganic material with ion exchange properties, based on tecto-alumosilicate, and described in U.S. Pat. No. 8,394,193 B2, proved to be highly suitable. These hardened tecto-alumosilicate binders may be applied like a fine mortar that hardens within 1-6 hours and exhibits ion-exchange properties and therefore good ion-conductive properties that are much less dependent on ambient humidity than the electrolytic resistivity of concrete. Typical electrolytic resistivities of the tecto-alumosilicate binders described in U.S. Pat. No. 8,394,193 B2 are 50 Ohm.m at 75% ambient rh and 200 Ohm.m at 50% rh. On the other hand, resistivities of standard concrete (w/c 0.5-0.45) are in the range of about 100-1500 Ohm.m at 80% ambient rh, 1000-100,000 Ohm.m in dry concrete at 50% rh, and in carbonated concrete about 10,000-15,000 Ohm.m [1, 2]. Therefore, anodes placed on or into concrete and whose electrolytic contact with the metal part that is to be protected, e.g. an anchor head, is made solely by concrete, may not deliver sufficient protective current or may fail completely under dry conditions. This applies especially to anodes applied on concrete surfaces, as the surface layer of the concrete may dry out rapidly and reduce the protective current delivered by the anode, whereas the metal part embedded into the concrete at a depth of 5 cm or more will still be in a humid environment and, in presence of chloride, will continue to corrode.

Therefore, by placing an ion-conducting electrolyte with a significantly lower electrolytic resistivity than the surrounding concrete, and which is significantly less sensitive to ambient humidity, between the anode and the metal part that is to be protected, one may assure that sufficient current reaches the sensitive metal. In accordance with certain aspects of an embodiment, the anode is preferentially a galvanic sacrificial anode. Good results have been obtained with anodes made from zinc and its alloys and aluminum and its alloys. Preferentially, the anode consists of a mesh, grid, perforated sheet, perforated plate, ribbons, perforated ribbons, or strands.

FIG. 2 is a schematic view of an exemplary configuration of an anode assembly in accordance with certain aspects of an embodiment of the invention. FIG. 2 shows a post-tensioned concrete member 20 in which the anchor head 22, the anchor plate 23 and the tensioned tendons 24 are embedded. The space between the outer surface of concrete member 20 and the anchor head 22 and anchor plate 23 is filled with tecto-alumosilicate binder 26 as described above. A preferably zinc-mesh anode 27, placed on the outer surface of the concrete member 20, is embedded into an embedding zinc activating binder 28, which is preferably similar or identical to the tecto-alumosilicate binder 26. The zinc-mesh anode 27 is electrically connected through, e.g., the soft steel reinforcement that is electrically connected to the tensioning assembly (i.e., anchor head 22, anchor plate 23, and tensioned tendons 24), or is directly connected to the steel anchor plate 23, e.g., via an electrical wire (not shown).

Tecto-alumosilicate binders are usually made of two reactive components—an alumo-silicate component and an alkaline activator component containing a soluble alkali-silicate. A suitable binder may comprise an aqueous suspension of an alumo-silicate or a mixture of alumo-silicates into which a highly alkaline (pH >14) alkali-silicate is mixed as an activator. The alumo-silicate component has a ratio of (CaO+MgO+Al₂O₃)/SiO₂>0.5, and more preferably >0.8. The binder sets and hardens in usually 0.5 hours to 6 hours, and more preferably within 1 hour, after mixing the two reactive components. To control consistency such as fluidity and shrinkage and strength, suitable fillers or aggregates, such as ground marble or quartz sand, may be added to the binder. The admixture of ground marble with a grain size distribution of 0.1-1 mm in a ratio of 1.5:1 of binder/filler resulted in a highly suitable fluid mortar with a fluidity of 160 mm according to EN 1015-3. To allow the application of the binder on vertical concrete members or overhead, the additives that increase thixotropy may be admixed, e.g., to the alumo-silicate component. Suitable additives are, e.g., based on cellulose-ethers with a molecular weight ranging from 1000 to 100,000, more preferably from 5,000 to 25,000. An admixture of glass fibers proved to be highly advantageous in controlling shrinkage and crack formation.

In accordance with certain aspects of an embodiment of the invention, the galvanic current flows from the zinc-mesh anode preferentially and selectively to the anchor-head and the anchor-plate, and additionally protects parts of the tendons D that are eventually exposed to chloride. This anode assembly has the advantage that the anchor head and anchor plate are protected preferentially from corrosion by directing the galvanic current towards these sensitive parts and additionally protecting the tensioned tendons without interfering at all with the concrete structure.

In accordance with further aspects of an embodiment of the invention, the system and method described herein may also provide the advantage of protecting metal-part assemblies that are composed of different types of metals or metal alloys that are electrically connected to each other. Usually, if different metals are connected and embedded into a material that functions as an electrolyte, such as concrete, then a galvanic element is formed. The less noble metal acts as an anode and corrodes. Examples include, e.g., anchor heads made from cast iron that are directly connected to tendons made from high strength steel alloyed with manganese, forming a galvanic couple. The anode assembly shown in FIG. 2 will eliminate such galvanic elements, preventing corrosion induced by the contact of different metals.

FIG. 3 is a schematic view of an exemplary configuration of an anode assembly in accordance with further aspects of an embodiment of the invention. In FIG. 3, the contact zone of the anchor head 22 and anchor plate 23 with the ion-conductive material 26 is covered or coated with an alkaline cement 30 (>pH 11), preferably Portland cement Mortar, allowing the use of ion-conductive materials with pH<11.

The present invention is not only applicable for new structures but also for structures already in use or for concrete members already cast and ready to be used. In that case, the grout cap over the anchor head and anchor plate (discussed above with respect to FIG. 1) may be removed by a suitable instrument or by high pressure water jetting. Subsequently, the ion-conductive material according to the invention may be placed and the anode assembly installed as described above and shown in FIG. 2 or 3.

Further and with regard to FIG. 4, one may use precast plugs 40 made from an ion-conductive material as discussed above, which plug 40 may be embedded into a suitable binder 26, preferably formed of the same or similar material as the plug. This configuration of the invention allows rapid installation of the anode assembly.

With reference to FIG. 5, one may embed into the plug 40 an anode 50, preferentially made from the same anode material as the anode 27 that is installed at the concrete member surface. In this configuration, the embedded anode 50 is equipped with a connecting wire, strand or ribbon 52 to connect the plug anode 50 to the surface anode 27. The embedded anode 50 is preferentially made from mesh, grid, perforated sheets or perforated plate to allow the ion current to flow freely between the surface anode 27 and the metal part that is to be protected. The zinc mesh, grid or perforated sheet may be folded ore that is wrapped to increase the amount of anode material without impeding the ion-current flow. The connecting wire, strand or ribbon consists of a suitable electric conductive material, preferably the anode material, galvanized steel, stainless steel, or titanium.

The additional anode inserted with the plug will prolong the service life of the anode assembly.

If only the selective protection of metal parts without additional protection of metal parts in the proximity is required, then one may install only the precast plug 40, electrically connecting the plug 40 to the metal part that is to be protected (FIG. 4) without installing the surface anode 27. During galvanic corrosion protection, the pH of the steel protected by the galvanic anode will increase due to the alkali hydroxides formed near the steel surface, assuring enhanced corrosion protection. However, to enhance safety and durability of corrosion protection, the electrolyte according to the present invention may contain at least one corrosion inhibitor, preferably in electrolytes with a pH<11.

Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It should be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.

REFERENCES

-   [1] K. Osterminsky, R. Polder, P. Schiessl, Long term behaviour of     the resistivity of concrete, HERON Vol. 57 (2012) No. 3, pp 211-230. -   [2] Petrica Ionut I. Banea, Study of Electrical Resistivity of     Mature Concrete, MSc Thesis, Delft University of Technology, Faculty     of Civil Engineering and Geosciences (Building Engineering) and TNO     Structural Reliability Department, Student number: 4125525, Delft,     June 2015. 

1. A system for the corrosion protection of metal parts embedded in a concrete member, comprising: a metal part embedded in said concrete member; a first anode; and an ion-conducting material between said metal part and said anode, said ion-conducting material exhibiting higher ionic conductivity than concrete embedding said metal part, and wherein said ion-conducting material is configured to direct a corrosion-protective current towards said metal part.
 2. The system of claim 1, wherein the first anode is a galvanic sacrificial anode.
 3. The system of claim 1, wherein the first anode is composed of zinc, zinc alloys, aluminum, or aluminum alloys.
 4. The system of claim 1, wherein the first anode is composed of a mesh, grid, perforated sheet, perforated plate, ribbons, perforated ribbons, or strands.
 5. The system of claim 1, wherein the ion-conducting material exhibits at least 20% higher conductivity than said concrete embedding said metal part.
 6. The system of claim 1, wherein the ion-conducting material exhibits at least 50% higher conductivity than said concrete embedding said metal part.
 7. The system of claim 1, wherein the ion-conducting material is composed of a tecto-alumo-silicate material.
 8. The system of claim 1, wherein the first anode is embedded into the ion-conducting material.
 9. The system of claim 1, wherein the ion-conducting material contains a corrosion inhibitor.
 10. The system of claim 1, wherein the metal part comprises steel or cast iron.
 11. The system of claim 1, wherein the metal part forms part of an assembly for pre- or post-tensioning of concrete members.
 12. The system of claim 1, wherein said ion-conducting material fills an opening in said concrete formed by carving out a portion of said concrete embedding said metal part.
 13. The system of claim 12, further comprising a precast plug formed from said ion-conductive material and inserted into said opening and embedded into a bonding ion-conductive material.
 14. The system of claim 13, wherein a galvanic anode is embedded in said precast plug.
 15. The system of claim 14, wherein said galvanic anode embedded in said precast plug further comprises a galvanic sacrificial anode.
 16. The system of claim 14, wherein said galvanic anode embedded in said precast plug is composed of zinc, zinc alloys, aluminum, or aluminum alloys.
 17. The system of claim 14, wherein said galvanic anode embedded in said precast plug is composed of a mesh, grid, perforated sheet, perforated plate, ribbons, perforated ribbons, or strands.
 18. The system of claim 14, wherein said galvanic anode is wrapped or folded.
 19. The system of claim 14, wherein said first anode extends along an exterior surface of said concrete member to provide a combined galvanic protection of metal parts embedded in said concrete member.
 20. The system of claim 1, wherein said first anode is electrically connected with single or multiple electrical connections for each said metal part.
 21. The system of claim 20, wherein the electrical connections are made by welding, mechanically joining, or gluing.
 22. The system of claim 21, wherein the electrical connections comprise a conductive metal wire, strap, rod, or bar.
 23. The system of claim 22, wherein the electrical connections further comprise copper, zinc, aluminum, titanium, steel, or stainless steel.
 24. The system of claim 20, wherein the electrical connections are made to any metallic member of a pre- or post-tensioned assembly that includes said metal part. 